be laughing up (one's) sleeve

To be trying to hide that one is laughing at someone else. It seems like the popular kids are laughing up their sleeves every time I walk by. It's so infuriating.

be laughing up your sleeve

OLD-FASHIONED

If someone is laughing up their sleeve, they are secretly amused by something, often something silly that someone else has done. They must be laughing up their sleeves at our stupidity. Note: The image here is of someone trying to hide the fact that they are laughing by putting their hand or arm in front of their mouth.